1. Field of the Invention
The present invention relates generally to microcellular communications terminals utilizing at least two antennas for reception of a signal and, more particularly, to an apparatus and method for selecting between a plurality of antennas having unequal gains to receive such signal.
2. Description of Related Art
Antenna diversity is a commonly used technique to overcome the effects of fading, which is typically the dominant impairment in wireless communication. The most common methods of performing diversity combining are maximal ratio combining (MRC), equal gain combining (EGC) and selection combining (SC). It has been found, however, that MRC and EGC require the co-phasing of the signals from the antennas before combining, which is difficult in practice.
While diversity combining can be performed either before or after detection of a signal, pre-detection selection combining, or pre-selection diversity, is preferred since it requires a terminal to have only a single receive chain and is therefore less expensive. At the same time, it will be understood that the signal format must allow sampling (or receipt) of the signal on the antennas prior to reception. Thus, pre-selection diversity is more suited to systems with slotted modes of transmission (e.g., TDMA systems). It will also be noted that pre-selection diversity is appropriate only when the fading rates on the signal are low enough to assume that the sample of the channel obtained at the beginning of the slot is valid for the duration of the entire slot. An example of a diversity receiving system is disclosed in U.S. Pat. No. 5,361,404 to Dent, which is also assigned to the owner of the present invention and is hereby incorporated by reference herein.
Pre-selection diversity has been utilized previously, but this process of determining the best performer at a given time has involved antennas with equal gains. One concern with the use of antenna diversity in a microcellular communications terminal is that the second antenna typically has a smaller gain than the main antenna. This oftentimes stems from the second antenna being located internal to the terminal for aesthetic purposes, where a user's hand wrapped around the terminal serves to reduce the gain of such second antenna. Since large gain differentials have been experienced (e.g., up to 4 dB in some cases), they must be taken into account or the receiving system will almost always choose the antenna having the greater gain even if the carrier power/interference ratio from the antenna having the lesser gain would otherwise have been larger.
Accordingly, it is a primary object of the present invention to provide an apparatus and method for selecting between a plurality of antennas utilized by a microcellular communications terminal for reception of a signal when such antennas have unequal gains.
It is another object of the present invention to provide an apparatus and method for selecting between a plurality of antennas utilized by a microcellular communications terminal for reception of a signal using preselection diversity.
It is still another object of the present invention to provide an apparatus and method for selecting between a plurality of antennas utilized by a microcellular communications terminal for reception of a signal in which the gain difference between such antennas is determined and/or continuously updated.
Yet another object of the present invention is to provide an apparatus and method for selecting between a plurality of antennas utilized by a microcellular communications terminal for reception of a signal which takes into account noise and interference encountered by the antennas.
These objects and other features of the present invention will become more readily apparent upon reference to the following description when taken in conjunction with the following drawings.